Backend: Add DISMISSED status and feature flag for status-based FP gating

What does this MR do and why?

Backend changes for Show FP badge for AI-detected flags regardless of confidence score, introducing the vulnerability_flag_status_based_gating feature flag and all backend support for status-based gating.

Frontend changes are in the follow-up MR: !230075

Key changes

Model:

• Add dismissed: 5 enum value to Vulnerabilities::Flag for manual dismissals
• Add DISMISSABLE_STATUSES constant (detected_as_fp, detected_as_not_fp)

Service:

• Update DismissFalsePositiveService to create flags with status: :dismissed (behind FF)
• Use status-based can_dismiss_flag? check (behind FF), falling back to confidence-score check

Helper:

• Serialize flag status (UPPER_SNAKE_CASE) and origin in vulnerabilities_helper.rb (behind FF)
• Add nil-safety guard for status&.upcase

Feature flag:

• Add vulnerability_flag_status_based_gating WIP feature flag
• Push feature flag in vulnerability controllers (project, group, vulnerability report)
• Rollout issue: #595697

GraphQL:

• Add DISMISSED value to VulnerabilityFalsePositiveDetectionStatus enum
• Regenerate introspection schema

How to set up and validate locally

1. Enable the feature flag:

   Feature.enable(:vulnerability_flag_status_based_gating)

2. Create test data:

   project = Project.find_by_full_path('YOUR_PROJECT_PATH')
   finding = project.vulnerabilities.first.finding
   
   finding.vulnerability_flags.create!(
     flag_type: :false_positive,
     origin: 'ai_sast_fp_detection',
     confidence_score: 0.0,
     status: :detected_as_not_fp,
     description: 'AI detected as not FP',
     project_id: project.id
   )

3. Verify in Rails console:

   helper = Class.new { include VulnerabilitiesHelper }.new
   allow(helper).to receive(:current_user).and_return(User.first)
   data = helper.vulnerability_finding_data(project.vulnerabilities.first)
   data[:latest_flag][:status] # => "DETECTED_AS_NOT_FP"
   data[:latest_flag][:origin] # => "ai_sast_fp_detection"

MR acceptance checklist

• This MR does not add a migration
• Backend changes have spec coverage
• Changes are behind a feature flag